The present invention relates in general to resistance measuring circuits and, more particularly, to measuring squib resistance to verify proper operation.
A squib is a detonator wire used for example to ignite the explosive charge that inflates an air bag in an automobile. The squib is a low resistance conductive wire with multiple coatings of an explosive material. At impact during an automobile accident, a large current flows through the squib, heats the wire, and ignites the explosive layers. This initial explosion sets off a secondary charge that inflates the air bag to protect the occupants.
The squib comes in a variety of resistance values, say from 1.0 ohms to 5.0 ohms. Any squib below 1.0 ohm is considered shorted and defective. Likewise, any squib above 5.0 ohms is considered open and defective. A monitoring circuit in the automobile continuously checks the squib resistance and reports values outside the acceptable range as a warning light on the console display.
In the prior art, the monitoring circuit injects current through the squib wire and measures the resulting voltage drop. The injection current is generally limited to about 30.0 milliamps because higher currents tend to degrade the squib's performance such as the time constant to detonation. Thirty milliamps injected into one ohm develops thirty millivolts across the squib. The squib voltage is typically amplified across a differential amplifier having a gain of say ten to increase the measured signal to usable levels. The amplified squib voltage may be converted to a digital signal for processing through a microprocessor, or processed through analog comparators to determine if the squib resistance is within the acceptable range.
It is desirable to maintain the accuracy of measuring squib resistance to say within .+-.10%. Unfortunately, the input offset voltage of the differential amplifier is about 3.0 millivolts which substantially takes all the error budget. In addition, there is error associated with the 10:1 resistor ratios around the amplifier due to temperature and process variation. To compound the problem, squib monitoring circuits often come in dual or quad packages to measure multiple squibs for both driver and passenger sides. The error in one resistance measuring amplifier is sufficiently large as to become the dominate factor in production yield loss. Adding multiple resistance measuring amplifiers to the same semiconductor die could adversely impact the production yield. For example, if one amplifier on a semiconductor die yields 90%, two amplifiers on one die would yield 81%, and four amplifiers on one die would yield 64%. To avoid poor yield, prior art designs have resorted to using a single resistance measuring amplifier and multiplexing in the various squib voltage readings. However, the analog multiplexers also introduce offset error which further reduces the accuracy of the readings.
Hence, a need exists for accurately measuring squib resistance to verify proper operation.